复杂多态实例

从编程角度理解多态的意义

理解多态：

  1. 首先在于和编译器“心有灵犀“：你可以在设计功能时更加自如挥洒，这也是通向高手的必经之路。
  2. 其次是在看源码的时候，可以和其他高手互相印证。

实例一：参数多态+特设多态（substrate/frame/executive/src/lib.rs）

这个例子是substrate的基础执行库，主要用到泛型结构体(参数多态) + 泛型约束(trait impl: 特设多态)

一个更加复杂的例子：来自substrate的lib.rs

 
#![allow(unused)]
fn main() {
/// Main entry point for certain runtime actions as e.g. `execute_block`.
///
/// Generic parameters:
/// - `System`: Something that implements `frame_system::Config`
/// - `Block`: The block type of the runtime
/// - `Context`: The context that is used when checking an extrinsic.
/// - `UnsignedValidator`: The unsigned transaction validator of the runtime.
/// - `AllPalletsWithSystem`: Tuple that contains all pallets including frame system pallet. Will be
///   used to call hooks e.g. `on_initialize`.
/// - `OnRuntimeUpgrade`: Custom logic that should be called after a runtime upgrade. Modules are
///   already called by `AllPalletsWithSystem`. It will be called before all modules will be called.
pub struct Executive<
	System,
	Block,
	Context,
	UnsignedValidator,
	AllPalletsWithSystem,
	OnRuntimeUpgrade = (),
>(
	PhantomData<(
		System,
		Block,
		Context,
		UnsignedValidator,
		AllPalletsWithSystem,
		OnRuntimeUpgrade,
	)>,
);

impl<
		System: frame_system::Config + EnsureInherentsAreFirst<Block>,
		Block: traits::Block<Header = System::Header, Hash = System::Hash>,
		Context: Default,
		UnsignedValidator,
		AllPalletsWithSystem: OnRuntimeUpgrade
			+ OnInitialize<System::BlockNumber>
			+ OnIdle<System::BlockNumber>
			+ OnFinalize<System::BlockNumber>
			+ OffchainWorker<System::BlockNumber>,
		COnRuntimeUpgrade: OnRuntimeUpgrade,
	> ExecuteBlock<Block>
	for Executive<System, Block, Context, UnsignedValidator, AllPalletsWithSystem, COnRuntimeUpgrade>
where
	Block::Extrinsic: Checkable<Context> + Codec,
	CheckedOf<Block::Extrinsic, Context>: Applyable + GetDispatchInfo,
	CallOf<Block::Extrinsic, Context>:
		Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
	OriginOf<Block::Extrinsic, Context>: From<Option<System::AccountId>>,
	UnsignedValidator: ValidateUnsigned<Call = CallOf<Block::Extrinsic, Context>>,
{
	fn execute_block(block: Block) {
		Executive::<
			System,
			Block,
			Context,
			UnsignedValidator,
			AllPalletsWithSystem,
			COnRuntimeUpgrade,
		>::execute_block(block);
	}
}

impl<
		System: frame_system::Config + EnsureInherentsAreFirst<Block>,
		Block: traits::Block<Header = System::Header, Hash = System::Hash>,
		Context: Default,
		UnsignedValidator,
		AllPalletsWithSystem: OnRuntimeUpgrade
			+ OnInitialize<System::BlockNumber>
			+ OnIdle<System::BlockNumber>
			+ OnFinalize<System::BlockNumber>
			+ OffchainWorker<System::BlockNumber>,
		COnRuntimeUpgrade: OnRuntimeUpgrade,
	> Executive<System, Block, Context, UnsignedValidator, AllPalletsWithSystem, COnRuntimeUpgrade>
where
	Block::Extrinsic: Checkable<Context> + Codec,
	CheckedOf<Block::Extrinsic, Context>: Applyable + GetDispatchInfo,
	CallOf<Block::Extrinsic, Context>:
		Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
	OriginOf<Block::Extrinsic, Context>: From<Option<System::AccountId>>,
	UnsignedValidator: ValidateUnsigned<Call = CallOf<Block::Extrinsic, Context>>,
{
    ...
}
}